1. Field of the Invention
This invention relates to the dissipation of heat from a circuit board having bare silicon chips mounted thereon.
2. Description of Related Art
Circuit boards having mounted thereon electronic components such as integrated circuit chips are of course well known. Frequently the components generate sufficient heat so that a heat dissipation means must be provided. One means is simply a fan blowing air across the circuit board. Another means is a heat sink thermally connected to the circuit board (or the components). The heat sink may be positioned either on the front side of the circuit board (the side bearing the components) or the back side (the side opposite the one bearing the components). A thermally conductive material may be used to establish thermal contact between the heat sink and the heat-producing components and/or the circuit board. Illustrative disclosures relating to heat dissipation in circuit boards include Cipolla et al., U.S. Pat. No. 5,268,815 (1993); Kim et al., U.S. Pat. No. 5,552,635 (1996); and Shuff, U.S. Pat. No. 5,812,374 (1998).
There have been recent developments involving circuit boards in which the components are bare silicon chips. Bare silicon chips are chips in which the exposed surface is silicon unprotected by molded plastic packaging (although the chip may have a thin passivating or protective layer). Bare silicon chip technology is also known as Direct Chip Attach or DCA technology and is further described in publications such as Electronic Packaging and Production, pp. 12-20 (NEPCON West ""99). An example is DRAM chip sets developed by Rambus Inc., of Mountain View, Calif. Because these chips consume more power than a typical memory chip, a more aggressive heat dissipation design is required. For example, if a fan alone is used for heat dissipation, a ducted fan with extremely high flow rate is required, at a cost of increased power consumption and noisiness. In other designs, two fans are used, one for cooling the microprocessor chip and a separate, dedicated one for the DRAM chips. At the same time, unpackaged nature of bare silicon chips makes them more vulnerable to damage if proper care is not taken.
Raychem Corporation, of Menlo Park, Calif., has sold internally supported thermally conductive gel materials as interface materials for heat dissipation in circuit boards. Such materials are depicted in FIGS. 1a and 1b of allowed copending, commonly assigned application of Mercer et al., Ser. No. 08/746,024, filed Nov. 5, 1996. A customer purchases the supported gel material and affixes it to his own circuit board or heat sink. The internal support, in the form of a fiberglass mat embedded within the gel composition, is necessary to provide the necessary handleability; otherwise, the gel composition is too soft, tacky, and fragile. However, the support increases the compression modulus of the gel article, so that mechanical stresses are undesirably transmitted to the underlying electronic components.
Thus, it is desirable to develop a method for dissipating heat from bare silicon chip containing circuit boards, while at the same time protecting them from mechanical stresses or avoiding the transmission of such stresses to the bare silicon chips.
We have made an invention which effectively dissipates heat from bare silicon chip circuit boards while at the same time physically protecting the chips. Accordingly, we disclose a method of making a heat dissipation arrangement for a circuit board having plural bare silicon chips mounted thereon, comprising the steps of:
(a) providing a circuit board having a plural bare silicon chips mounted thereon, the plural bare silicon chips each having an exposed face;
(b) providing a heat spreader having a substantially planar portion having inner and outer faces and being sized and shaped for attachment to the circuit board such that the inner face of the planar portion faces the side of the circuit board bearing the bare silicon chips;
(c) applying to the inner face a precursor composition curable to a gel composition having a cohesive strength greater than its adhesive strength, a compression modulus less than 1.38 MPa, and a thermal conductivity greater than 1.0 W/m-xc2x0C.;
(d) curing the precursor composition to convert the precursor composition to the gel composition such that the gel composition forms at least one pad having a thickness between about 0.08 mm and about 1.0 mm; the at least one pad being positioned to contact and completely cover the exposed faces of the plural bare silicon chips when the heat spreader is attached to the circuit board; and
(e) attaching the heat spreader to the circuit board so that the at least one pad of gel composition contacts and completely covers the exposed faces of the plural bare silicon chips.
In another embodiment of the invention, there is provided a combination assemblable into a circuit board arrangement having provision for heat dissipation, comprising:
a circuit board having a plural bare silicon chips mounted thereon, the plural bare silicon chips each having an exposed face;
a heat spreader spaced apart from the circuit board and having a substantially planar portion having inner and outer faces and being sized and shaped for attachment to the circuit board such that the inner face of the planar portion faces the side of the circuit board bearing the bare silicon chips;
a gel composition on the inner face of the heat spreader, the gel composition having a cohesive strength greater than its adhesive strength, a compression modulus less than 1.38 MPa, and a thermal conductivity greater than 1.0 W/m-xc2x0C.; the gel composition forming at least one pad having a thickness between about 0.08 mm and about 1.0 mm; the at least one pad being positioned to contact and completely cover the exposed faces of the plural bare silicon chips when the heat spreader is attached to the circuit board.